This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. ABSTRACT The National Health and Nutrition Examination Surveys, NHANES II and III, report an increasing prevalence of overweight and obesity as well as of obesity related diseases in children. This is a serious public health problem necessitating research to determine which factors lead to obesity and which strategies could be used to prevent and treat obesity and its consequences. Most of the research conducted to address these issues requires comparison between obese and lean subjects. In order to provide accurate conclusions, the definition of "lean" and "obese" must be absolutely clear. The most routinely used criteria to distinguish lean from obese subjects is body mass index (BMI) i.e. weight in kg/ height in m2. However, a BMI within the normal range for age might result in heterogeneous groups, including subjects with both low and high body fat percentage, low and high visceral fat and low and high hepatic fat content. All of these factors might confound results and conclusions. In this protocol we will explore whether there is a correlation between body fat percentage (measured by DXA), visceral adiposity (measured by MRI), hepatic fat content (measured by MRI) and insulin sensitivity (assessed by fasting insulin and an Oral Glucose Tolerance Test (OGTT)) in adolescents with normal BMI (below the 85th percentile i.e. 25 kg/m2 in the planned age group) and body fat percentage equal to or above 27% in girls and 25% in boys. These results will be compared with those of our ongoing study (H-11954) including strictly lean and obese subjects Collectively the information from this proposed protocol and the ongoing study, will enable us to define the influence of BMI, body fat percentage and fat distribution on insulin sensitivity and glucose homeostasis in adolescents, which will in turn provide guidelines for optimizing screening methods in studies on obesity.